Share Capital - Restricted cash units (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares	1,826	2,559	0
Aggregate grant date fair value	$ 63,261	$ 94,525	$ 0
Compensation expense	39,433	19,834	0
Estimated tax benefit (charge)	8,543	$ 4,374	$ 0
Total unrecognized stock based compensation expense	$ 74,100
Period for recognition of unrecognized expense	1 year 6 months
Unvested at beginning of period, shares	2,006
Vested, shares	(892)
Forfeited, shares	(367)
Unvested at end of period, shares	2,573	2,006
Unvested at beginning of period, weighted average grant date fair value	$ 36.95
Granted, weighted average grant date fair value	34.64
Vested, weighted average grant date fair value	36.93
Forfeited, weighted average grant date fair value	36.27
Unvested at end of period, weighted average grant date fair value	$ 35.41	$ 36.95
Deferred compensation, share-based payments
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares	1,800
Aggregate grant date fair value	$ 63,200
Deferred compensation, share-based payments | Three Year Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period required	3 years
Deferred compensation, share-based payments | Two Year Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period required	2 years